1. Field of the Invention
The present invention relates to a color image processing apparatus having a function of detecting the position and size of an original, and more particularly, to the structure of detecting the position and size of an original by detecting a contrast between a portion where the original is present and a portion where the original is absent in, for example, a digital color copying machine.
2. Related Background Art
Original reading apparatuses for detecting the position and size of an original present on an original base are disclosed in, for example, U.S. Pat. No. 5,001,574.
In the technique disclosed in U.S. Pat. No. 5,001,574, image data is obtained by a line sensor which serves as image reading means and which employs a CCD for converting an original into an electric signal. Also, discrimination between an original area and a non-original area is made by obtaining a signal corresponding to black from the non-original area and by detecting a white base of the original. Furthermore, a counter for counting the number of output bits in a main scan direction of the line sensor and a counter for counting the number of lines in a sub scan direction are operated synchronously with a reference position of the original base. The position of the original base where the original is present is detected by detecting the white signal of the original area and by holding the counted values in the main and sub scan directions.
FIG. 2 explains how an original is read conventionally. In FIG. 2, reference symbols S and M respectively denote an original base and an original placed on the original base S. `x` indicates the main scan direction of the line sensor. `y` shows the sub scan direction thereof. SP is reference position with respect to the main scan direction x. The position and size of the original M placed on the original base S can be recognized by detecting a white signal of point P1 on the original M which is to be detected first, a white signal of point P2 closest to the reference position SP with respect to the main scan direction x, a white signal of point P3 farthest from the reference position SP with respect to the main scan direction x, and a white signal of point P4 which is to be detected last.
Conventional original reading techniques are also disclosed in U.S. Pat. Nos. 4,439,790, 4,811,047 and 4,686,577 and U.S. patent application Ser. No. 575,878.
However, in the aforementioned original reading techniques, since discrimination between the original area and the non-original area is made utilizing only a difference in the lightness of both areas, if the original has at its end portion a portion having a relatively low lightness, for example, a red portion, the original area may not be detected correctly, resulting in erroneous determination.
Such a problem occurs regardless of the material or color of an original cover for pressing the original.
Particularly, in the case of a full-color copying machine in which color originals are copied, the aforementioned problem is serious.
The same problem occurs in a color reader or the like.